Sand Grains: Chips Off The Old Rock

One of the most impressive feats of nature is the erosion of massive mountains into sand by the powerful forces of wind and water. Individual sand grains are the size of table salt grains (less than one millimeter in diameter), and resemble miniature gemstones when magnified. Sand from granitic mountains is often made of angular pieces of quartz, feldspar and mica. Strong winds pile the minute grains of desert lake beds and river deltas into large hills of sand called dunes. After countless centuries of rubbing together, the grains of some sand dunes become smooth and rounded, like tiny pebbles in a rushing stream. When these polished grains slide over each other, strange humming or booming sounds are sometimes produced, like a telegraph wire or squadron of World War II fighter planes. Sliding down a steep booming dune with an avalanche of sand is like riding down a department store escalator, ankle deep in sand. As the sand begins to vibrate, the humming sound gets louder and louder.

But not all sand grains are made of quartz and feldspar. The brilliant snow-white dunes of White Sands, New Mexico are composed of gypsum, and the spectacular black sand beaches of tropical Pacific islands are made of fine volcanic particles.

A black, volcanic sand beach on the Polynesian island of Tahiti.

Gleaming white sands of tropical coral beaches and atolls are the most beautiful of all. High magnification of the grains reveals a glistening, microscopic assortment of reef animals, including wave-worn fragments of brightly-colored corals, shells of minute one-celled protists called foraminiferans, fragments of seashells and shiny, star-shaped sponge spicules.

A magnified view of the tropical beach sand from the Caribbean island of St. John (U.S. Virgin Islands). The grains include porous fragments of brightly-colored corals, minute foraminiferan shells, fragments of sea shells and shiny, star-shaped sponge spicules.

A large percentage of the sand grains of some tropical beaches (such as Belize) come from minute fragments of calcareous green algae, including Halimeda opuntia. At least nine species of Halimeda are known in Caribbean waters, often growing among turtlegrass meadows on sandy bottoms or among luxuriant coral reefs. The upright branches of this unusual green alga are composed of segmented calcareous plates which become dusted with sediments. The segments superficially resemble a string of tiny wingnuts. The body (thallus) of this alga is firmly attached to mud, sand or rocky bottoms by a large holdfast. Microscopic jointed plates of dead Halimeda get washed ashore and become a significant portion of the calcium carbonate sand on tropical shores.

The lovely coral sand beaches of Belize are a delight to beachcombers looking for exotic drift seeds washed ashore from distant rain forests. A large percentage of the sand grains are composed of fragments from the green alga Halimeda.

A magnified view of tropical beach sand from Belize. The flattened grains are fragments from jointed calcareous plates of the green alga Halimeda (probably H. opuntia). This interesting green alga grows among the submarine turtlegrass beds and the luxuriant coral reefs of this region.